The capability of estimating available capacity, end-to-end, over a data transfer path of a data communication system comprising a data network is useful in several contexts, including network monitoring and server selection. Passive estimation of available capacity of a data transfer path, such as estimation of bandwidth of an end-to-end data transfer path is possible in principle, provided all network nodes in the data transfer path can be accessed. However, this is typically not possible, and estimation of available end-to-end capacity of the data transfer path, is typically done by active probing of the data transfer path. The available capacity such as bandwidth can be estimated by injecting data probe packets into the data transfer path, and then analyzing the observed effects of cross traffic on the probe packets. This kind of active measurement requires access to sender and receiver hosts, typically data network nodes, only, and does not require access to any intermediate nodes in the data transfer path between the sender and receiver nodes.
In the art, methods such as BART (“Bandwidth Available in Real Time”) and TOPP (“Trains of Packet Pairs”) have been developed for estimating capacity in a network. These methods comprise the sending of a sequence of packets, also known as a probe train, from a transmitting node to a receiving node. The probe train is sent at a specific rate depending on packet size and time interval between the packets in the train. If the send rate of the train is greater than the available path capacity (APC) of the network path, there will be a transient congestion at a bottleneck link. Due to the congestion, the probe-train packets will be buffered in the node just before the bottleneck link which will spread out the packets in time. The probe train will therefore be received by the receiving node at a lower rate than the send rate. See the document “Probing-Based Approaches to Bandwidth Measurements and Network Path Emulation” by Bob Melander, PhD Thesis, Uppsala University, 2003, for a description of TOPP and European Patent 1952579 for a further description of the BART method.
Further, the IETF IP Performance Metrics (IPPM) working group has defined two IP active measurement protocols: One-Way Active Measurement Protocol (OWAMP), RFC 4656, and Two-Way Active Measurement Protocol (TWAMP), RFC5357. OWAMP is designed for measuring one-way packet delay and one-way packet loss between two hosts. TWAMP is based on OWAMP. TWAMP is designed for measuring one-way and two-way (round-trip) packet delay and packet loss between two hosts.
In many networks, Quality of Service, QoS, mechanisms are included. Recent studies have shown that available capacity measurement methods may produce erroneous estimates in networks where QoS mechanisms are deployed, see e.g. “Available Bandwidth Measurements in QoS Environments” by Mark Bechtel and Paraskevi Thanoglou, Master Thesis, KTH, 2010.
Examples of such QoS mechanisms are rate limiting mechanisms such as a traffic shaper and a traffic policer, and these are often implemented as a token bucket, A token bucket is an algorithm used in packet switched computer networks and telecommunications networks to check that data transmissions conform to defined limits on bandwidth and burstiness (a measure of the unevenness or variations in the traffic flow). The token bucket algorithm is based on an analogy of a fixed capacity bucket into which tokens, normally representing a unit of bytes or a single packet of predetermined size, are added at a fixed rate. When a packet is to be checked for conformance to the defined limits, the bucket is inspected to see if it contains sufficient tokens at that time. If so, the appropriate number of tokens, e.g. equivalent to the length of the packet in bytes, are removed (“cashed in”), and the packet is passed, e.g., for transmission. If there are insufficient tokens in the bucket the packet does not conform and the contents of the bucket are not changed. Non-conformant packets can be treated in various ways:                They may be dropped.                    They may be enqueued for subsequent transmission when sufficient tokens have accumulated in the bucket.            They may be transmitted, but marked as being non-conformant, possibly to be dropped subsequently if the network is overloaded.                        
A conforming flow can thus contain traffic with an average rate up to the rate at which tokens are added to the bucket, and have a burstiness determined by the depth of the bucket, This burstiness may be expressed in terms of either a jitter tolerance, i.e. how much sooner a packet might conform (e.g. arrive or be transmitted) than would be expected from the limit on the average rate, or a burst tolerance or maximum burst size, i.e. how much more than the average level of traffic might conform in some finite period.
Thus, in short, if a token bucket or any other rate limiting mechanism is provided in a network, this may have a serious effect on available capacity measurements.